In the all-tungsten ASDEX Upgrade tokamak nitrogen seeding is a reliable method for radiative cooling at the plasma edge, regularly applied in high power scenarios. Interestingly, in the presence of nitrogen seeding the energy confinement is observed to improve significantly, compared with similar unseeded discharges, with an increase by 10–25% in plasma stored energy and HIPB98(y,2). In this paper, we document the improvement and we analyse the transport properties in the core plasma, by comparing similar discharges with and without nitrogen seeding. The increase in the suprathermal energy content is assessed as well. The impurity nitrogen is shown not to penetrate significantly into the core plasma. Non-linear gyro-kinetic simulations predict that the improvement observed in the pedestal confinement is transferred to the core via profile stiffness, in agreement with the experimental evidence, whereas the direct contribution from core confinement is small.
Plasma confinement at JET